Continuously cast acrylic sheet is prepared from methyl methacrylate monomer in two stages. First monomer is partially polymerized to obtain a syrup containing both monomer and polymer. Usually, the syrup contains about 20-30% polymer. The syrup is then cast into sheet form and the polymerization continued. This two stage polymerization is preferred because polymerization in the cast operation proceeds at a faster rate than if polymerization proceeded from a monomer mix alone.
Two types of acrylic sheet are prepared by the foregoing procedure. A so-called "general purpose" sheet having a high molecular weight is prepared by carrying out both stages of the polymerization in the absence of a chain transfer agent (i.e., an agent which terminates growth of one polymer chain and starts growth of another chain). The other type of sheet is a so-called "thermoformable" sheet which is prepared by employing a chain transfer agent and a crosslinking agent. The chain transfer agent keeps the molecular weight of the polymer low which is necessary for thermoformability, thus it is added to the monomer mix used to prepare syrup. The crosslinking agent provides a means for obtaining a crosslinked network of high molecular weight and it is ordinarily added to the syrup.
Because the chain transfer agent is present during preparation of the syrup for thermoformable sheets, the syrup cannot then be used to prepare general purpose sheets. Furthermore, the syrup preparation and storage equipment must be cleaned every time it is desired to switch from preparation of syrup for thermoforming sheet to syrup for general purpose sheet. It would be desirable to be able to employ one syrup that could be used to prepare both thermoformable sheet or general purpose sheet.